The high data communication rates required of modern communication systems often require point-to-point electromagnetic transmission. Because of the high data rates now in common use, the bandwidths of the signals to be transmitted may be large. In order to accommodate these bandwidths, it has become common to modulate the signals to be transmitted onto microwave carriers and to transmit the signals at microwave or millimeter-wave frequencies corresponding to about three gigahertz (GHz) and above. A major problem with point-to-point communication systems of this sort is occasional loss of signal or fading due to multipath propagation arising from anomalous atmospheric conditions. For many applications, the resulting interruption of communication is undesirable, and several methods have been used to ameliorate the condition. One method for combatting fading in a point-to-point communication scheme is to use frequency diversity. In frequency diversity, the same data is modulated onto carriers of different frequencies, and transmitted simultaneously over the same path. At the receiver, the received signals are demodulated, and the strongest or otherwise best demodulated signal is routed to the utilization apparatus. While very effective, frequency diversity tends to be wasteful of frequency spectrum. Dual frequency diversity, for example, results in a doubling of spectrum use. For narrowband signals, this may not be a problem. When several signals having a bandwidth on the order of 1 GHz are to be transmitted, the spectrum which must be allocated in a frequency diversity context may not be available.
Another method for combating fading is known as space diversity. Space diversity systems are often implemented as receiver diversity systems, which means that the point-to-point communication system includes a single transmitting antenna transmitting the modulated signal towards a pair of receive antennas. Each receive antenna receives the transmitted signal by a somewhat different path, and the amount of fading differs by each path. The best or strongest demodulated signal is routed to the utilization apparatus, thereby ameliorating the effect of fading. This type of system is effective, especially if the receive antennas are spaced from each other sufficiently so that the signals arriving at the one antenna are uncorrelated with the signals arriving at the other antenna. These considerations are discussed in detail in an article entitled "Space-Diversity Engineering" by A. Vigants, published at pp. 103-142 of the Bell System Technical Journal, Volume 54, No. 1, Jan. 1975. According to Vigants, transmitting diversity is disadvantageous because of the vulnerability of the control signal. However, in communications by multiple hops between towers, alternate transmit and receive diversity can be used. Both transmitting and receiving diversity can be used together, according to Vigants.
A dual transmitting diversity communication system is desired in which the control signals are not vulnerable to fading.